Introduction to Polymer Science and Technology

Microstructure

Diffraction angle, 20

Figure 4.11Theintensity of the X-radiation scattered by the specimen vs. diffraction angle, showing Bragg diffraction peaks on an amorphous

background

Wide-angle X-ray scattering (WAXS), which produces the diffraction patterns shown in Figure 4.11, is used for the determination of crystalline fraction as well as crystalline dimensions. In determination of crystalline dimensions, WAXS is used for small-scale microstructure measurements (<1 nm) such as unit cells, while SAXS is used to investigate large-scale morphological features (1 to 1000 nm) such as the lamellar long period that was covered in Section 4.2. SAXS measurements are conducted at very small scattering angles (0.022 to 2.2°) and therefore require collimators to sharply focus the incident beam and more specialised detectors than required for WAXS.

Introduction to Polymer Science and Technology

Microstructure

A sketch of the scattering intensity vs. angle, Figure 4.11, is used to explain the crystallinity determination.

Scattering (or diffraction) angle, 28

photographic plate

specimen

Figure4.12The intensity of theX-radiation scattered by the specimen vs. scattering angle The areas indicated in Figure 4.12 are described below:

a) scattering due to the amorphous component of the polymer

b) background scattering (air, dust, specimen mount)

c) scattering due to the crystalline component of the polymer.

Consider that equal masses of different substances scatter equal amounts of radiation, thus:

Crystalline fraction, x = I / (I + I), where I and I are the scattered radiation intensities (i.e., areas under the respective

regions on the graph).

4.3.3 Infra-red method

Infrared (IR) spectroscopy is also employed to determine crystallinity because the vibrations of the atoms detected by IR spectrometer are affected by the crystalline structure and appear on the spectrum at slightly higher energy levels than do more freely moving atoms in the amorphous regions.

Figure 4.13 shows a portion of the IR spectrum for semicrystalline and amorphous polypropylene specimens. On IR spectra there are crystalline, amorphous and crystalline-amorphous sensitive bands. For PP, the crystalline sensitive band is at 10.03 (xm wavelength, and both the crystalline and amorphous sensitive band is at 10.29 |xm.

% crystallinity « A(10.03 |xm) / A(10.29 |xm) where, A (X) is the absorbance at the wavelength X.

Date: 2015-12-11; view: 745